The invention concerns the production of laminates, in particular the continuous production of laminates from fibers reinforced high temperature thermoplastics materials.
Throughout this specification the term laminate is intended to encompass a structure formed by consolidating, by application of heat and pressure, one or more layers of fibers reinforced material impregnated with one or a mixture of thermoplastics materials to form a laminate of desired shape. The polymers may have fillers or other additives added to them to give desired strength and/or visual characteristics to the final laminate. The laminate may have additional outer (or inner) layers of other materials for decorative and/or functional purposes.
Throughout this specification the term high temperature thermoplastics material is intended to encompass thermoplastics materials having a processing temperature of 280.degree. C. or above. Such thermoplastics materials include polysulphones, polyethersulphones, polyetheretherketones, polyetherimides and polyimides.
Throughout this specification the term glass transition temperature is intended to encompass both the glass transition temperatures of crystalline polymers and the heat distortion/softening temperatures of amorphous polymers.
The production of laminates from fiber reinforced low temperature thermoplastics materials (such as polypropylene) is well known and normally when making continuous lengths of them the heating and compression of the impregnated fibers reinforcement is effected by passing the impregnated fibers reinforcement through one or more pairs of heated compression rollers. In this way the polymer material is forced into the interstices of the fibers reinforcement, is cured and a laminate of the desired form produced. This has good results with the low temperature thermoplastics materials noted which can be processed at temperatures of up to 270.degree. and pressures of 10-50 p.s.i.
A difficulty found with this technique for producing laminates is in controlling the consistency of the final product. It has been found that laminates produced by passing thermoplastics impregnated fibers reinforcement through compression rollers may be formed with "dry" areas--that is to say areas in which the polymer has not thoroughly wetted the fibers reinforcement. This problem appears to be particularly associated with the use of compression rollers and is exacerbated when attempting to use the technique with high temperature thermoplastics materials. Attempts to overcome the difficulties have included another proposal for continuously forming laminates, which makes use of a compression tool having a pair of platens between which the thermoplastics material impregnated fibers reinforcement is passed in steps--the tool being operated between step movements of the impregnated fibers reinforcement to compress it and form the laminate.
This proposal partially overcomes the noted difficulty found with the use of compression rollers, however, the laminates produced (especially from high temperature thermoplastics materials) still include some areas which are "dry", and additional problems which arise are the misalignment of the fibers reinforcement and the formation of "voids" in the finished product. These problems can usually become overcome by correct adjustment and calibration of the equipment (e.g. temperature, pressure and cycle time) and by the proper design of the feed and guide parts of the apparatus.
Additionally, it has been suggested that the impregnated fibers reinforcement be moved through the compression tool in small steps such that each area of the impregnated fibers reinforcement is subjected to multiple compressions within the tool.
This has had limited success in eliminating the visual imperfections in the final laminate and has done nothing for (or even exacerbated) another problem found when producing laminates, namely the sticking of the thermoplastics impregnant to the surfaces of the tool (compression rollers or platens) used, which leads to difficulty in controlling the pressure applied to the laminate within the tool and consistency of the final product.
To prevent this happening it has been suggested that the surfaces of the tool contacting the impregnated fibers reinforcement in the tool be coated with a release agent (e.g. P.T.F.E.) which acts to inhibit adhesion of the polymer in the impregnated fibers reinforcement to the surfaces of the tool.
However, even with this safeguard the polymer material can still adhere to the surfaces of the tool with the effect that the laminates produced are imperfect and with the result that the process must be stopped to allow the tool to be broken down, cleaned, re-coated with the release agent and reassembled to enable laminate production to continue. Such a procedure is expensive both in terms of labour and of lost laminate production.
These problems of processing high temperature thermoplastics materials are not, of course, met when making laminates in a closed mould--in the piece--in which circumstance the temperature of the workpiece and the pressure applied to the workpiece may be controlled as desired.